Slater Insulator in Iridate Perovskites with Strong Spin-Orbit Coupling

The perovskite ${\mathrm{SrIrO}}_3$ is an exotic narrow-band metal owing to a confluence of the strengths of the spin-orbit coupling (SOC) and the electron-electron correlations. It has been proposed that topological and magnetic insulating phases can be achieved by tuning the SOC, Hubbard interactions, and/or lattice symmetry. Here, we report that the substitution of nonmagnetic, isovalent ${\mathrm{Sn}}^{4+}$ for ${\mathrm{Ir}}^{4+}$ in the ${\mathrm{SrIr}}_{1-x}{\mathrm{Sn}}_x{\mathrm{O}}_3$ perovskites synthesized under high pressure leads to a metal-insulator transition to an antiferromagnetic (AF) phase at $T_N\ge 225\mathrm{K}$. The continuous change of the cell volume as detected by x-ray diffraction and the $\lambda$-shape transition of the specific heat on cooling through $T_N$ demonstrate that the metal-insulator transition is of second order. Neutron powder diffraction results indicate that the Sn substitution enlarges an octahedral-site distortion that reduces the SOC relative to the spin-spin exchange interaction and results in the type-$G$ AF spin ordering below $T_N$. Measurement of high-temperature magnetic susceptibility shows the evolution of magnetic coupling in the paramagnetic phase typical of weak itinerant-electron magnetism in the Sn-substituted samples. A reduced structural symmetry in the magnetically ordered phase leads to an electron gap opening at the Brillouin zone boundary below $T_N$ in the same way as proposed by Slater.

Figure 1

Temperature dependence of the magnetization $M/H$, resistivity $\rho$, and the thermoelectric power $S(T)$ for ${\mathrm{SrIr}}_{1-x}{\mathrm{Sn}}_x{\mathrm{O}}_3$ ($0\le x\le 0.3$) samples. Resistivity curves on cooling down and warming up overlap. The right label is for the magnetization and the inside label is for thermoelectric power. Arrows point to the transition temperature as detected by the change of thermoelectric power.

Figure 2

(a) The bond lengths in an $\mathrm{Ir}(\mathrm{Sn}){\mathrm{O}}_6$ octahedron in ${\mathrm{SrIr}}_{1-x}{\mathrm{Sn}}_x{\mathrm{O}}_3$ resolved by neutron powder diffraction; the data for the $x=0$ sample are after Ref. [15]. The inset: schematic drawing of octahedra, their tilting configuration in the structure, and the relationship to the spin canting. (b) Temperature dependence of magnetic susceptibility for ${\mathrm{SrIr}}_{1-x}{\mathrm{Sn}}_x{\mathrm{O}}_3$.

Figure 3

(a), (b) Temperature dependences of lattice parameters for $x=0.1$ and 0.2 samples of ${\mathrm{SrIr}}_{1-x}{\mathrm{Sn}}_x{\mathrm{O}}_3$ determined by x-ray diffraction; dashed lines indicate the MI transition temperatures; (c) $Q$ dependence of neutron powder diffraction intensity measured at 100 K (circles) and 300 K (diamonds), normalized to counts per minute (cpm) for the $x=0.2$ sample. (d) Temperature dependence of intensity measured at the fixed position $Q=1.377{\AA }^{-1}$. The line is a guide to the eye.

Figure 4

(a) XAS and XMCD spectra of ${\mathrm{SrIr}}_{1-x}{\mathrm{Sn}}_x{\mathrm{O}}_3$ ($0\le x\le 0.5$) samples at 150 K, and (b) the phase diagram of ${\mathrm{SrIr}}_{1-x}{\mathrm{Sn}}_x{\mathrm{O}}_3$ together with the expectation value of spin-orbit coupling.